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An outbreak is occurring if the total number of cases of influenza or ILI on a cruise ship among passengers OR crew exceed the threshold of 1.380 cases per 1000 person-days, as calculated below. This threshold is based on Alaska regional data from 1998.*

Threshold case rate:

The burden of illness indicating a possible onboard ILI outbreak

Equivalent to 1.38 cases per 1,000 passenger-days

Variables (Passengers and crew are calculated separately):

Total passengers or crew members = (p)

Number of days onboard for the voyage = (d)

Passenger or crew days = p times d = (p X d)

Cumulative cases to reach threshold = (n)

Calculation:

n = (1.38)(p X d) ÷ 1,000

The number of actual cases reported should be greater than n to be considered an outbreak.

Examples

Example 1: How do you calculate the passenger threshold?

A ship with 1,200 passengers onboard for 21 days reports 30 passenger cases

p = 1200, d = 21

n = (1.38)(p X d)/1000 = (1.38)(1200 X 21)/1000 = 34.8 ≈ 35 cases

Since 35 is greater than 30, the threshold (n) has not been reached

Example 2: How do you calculate the crew member threshold?

The same ship with 800 crew members onboard for 21 days reports 25 crew member cases

p = 800, d = 21

n = (1.38)(p X d)/1000 = (1.38)(800 X 21)/1000 = 23.2 ≈ 23 cases

Since 23 is less than 25, the threshold (n) has been reached

*Bodnar UR, Maloney SM, Fielding KL, et al. Preliminary guidelines for the prevention and control of influenza-like illness among passengers and crew members on cruise ships. Atlanta: US Department of Health and Human Services, 1999.